Objective lens for optical pick-up
View Patent ↗There is provided an objective lens for an optical pick-up. The objective lens is formed to be a single-element plastic lens having a first surface and a second surface. The first surface is configured to have, within an effective diameter, an inflection point at which a second derivative of a sag of the first surface takes a value of 0. Further, the objective lens having a numerical aperture larger than or equal to 0.75.
1. An objective lens for an optical pick-up,
the objective lens being formed to be a single-element plastic lens having a first surface and a second surface,
the first surface being configured to have, within an effective diameter, an inflection point at which a second derivative of a sag of the first surface takes a value of 0,
the objective lens having a numerical aperture larger than or equal to 0.75, and
the sag being defined as a distance between a point on an aspherical surface at a height from the optical axis and a plane tangential to the aspherical surface at the optical axis.
2. The objective lens according to claim 1 ,
wherein the objective lens satisfies a condition:
0.94 <h ( x )≦0.99 (1)
where h(x) represents a height of the inflection point in a pupil with respect to an optical axis of the objective lens.
3. The objective lens according to claim 1 ,
wherein the objective lens satisfies a condition:
1.15 <f< 1.45 (2)
where f represents a focal length of the objective lens.
4. The objective lens according to claim 1 ,
wherein the objective lens satisfies a condition:
1.20<( SAG 1)′/ n< 1.65 (3)
where (SAG1)′ represents a first derivative of the sag of the first surface at the inflection point and n represents a refractive index of the objective lens.
5. The objective lens according to claim 1 ,
wherein the objective lens satisfies a condition:
−0.60<(( SAG 2)′/ n ) MIN ≦−0.10 (4)
where ((SAG2)′/n) MIN represents a minimum value of (SAG2)′/n, (SAG2)′ represents a first derivative of a sag of the second surface, and n represents a refractive index of the objective lens.
6. The objective lens according to claim 1 ,
wherein the objective lens satisfies a condition:
0.77 <d /( f·n )<1.10 (5)
where f represents a focal length of the objective lens, d represents a lens thickness of the objective lens, and n represents a refractive index of the objective lens.
7. The objective lens according to claim 1 ,
wherein the objective lens satisfies a condition:
−1.50 <r 1 /r 2<−0.60 (6)
where r1 represents a radius of curvature of the first surface, and r2 represents a radius of curvature of the second surface.
8. The objective lens according to claim 1 , wherein the first surface is located on a light source side and the second surface is located on an optical disc side.
9. The objective lens according to claim 1 ,
wherein the first surface has an anti-reflection coating formed thereon,
wherein the anti-reflection coating formed on the first surface satisfies a condition:
1.00 <Tp/Tc< 1.40 (7)
where Tc represents transmissivity of a circular central part defined within a range of 10 percent of the effective diameter with respect to a center of the first surface, and Tp represents transmissivity of a ring-shaped part defined outside of 90 percent of the effective diameter with respect to the center of the first surface,
wherein the objective lens satisfies a condition:
1.75<λ 0 /λ min <2.00 (8)
where λ 0 represents a wavelength at which reflectivity of the anti-reflection coating takes a minimum value for a beam incident perpendicularly on the anti-reflection coating, and λ min represents a minimum wavelength of wavelengths of laser beams to be used.
10. The objective lens according to claim 9 ,
wherein the objective lens has a refractive index lower than 1.58 with respect to the minimum wavelength λ min of all of the wavelengths of the laser beams to be used.
11. The objective lens according to claim 9 ,
wherein the anti-reflection coating formed on the first surface has a two-layer structure,
wherein the second surface has an anti-reflection coating formed thereon,
wherein the anti-reflection coating formed on the second surface has a two to four-layer structure.
12. The objective lens according to claim 1 , wherein the aspherical surface is expressed by the following equation:
X
(
h
)
=
Ch
2
1
+
1
-
(
1
+
κ
)
C
2
h
2
+
A
4
h
4
+
A
6
h
6
+
A
8
h
8
+
…
+
A
22
h
22
wherein X(h) represents the sag amount;
h is the height from the optical axis;
κ represents a conical coefficient;
A 4 , A 6 . . . A 22 are aspherical coefficients of 4 th , 6 th . . . 22 nd orders, respectively; and
C is curvature of the aspherical surface on the optical axis.
13. An optical system of an optical pick-up, comprising:
a light source which emits a light beam; and
an objective lens on which the light beam is incident,
wherein:
the objective lens is formed to be a single-element plastic lens having a first surface and a second surface;
the first surface is configured to have, within an effective diameter, an inflection point at which a second derivative of a sag of the first surface takes a value of 0;
the objective lens has a numerical aperture larger than or equal to 0.75; and
the sag is defined as a distance between a point on an aspherical surface at a height from the optical axis and a plane tangential to the aspherical surface at the optical axis.
14. The optical system according to claim 13 ,
wherein the objective lens satisfies a condition:
0.94 <h ( x )≦0.99 (1)
where h(x) represents a height of the inflection point in a pupil with respect to an optical axis of the objective lens.
15. The optical system according to claim 13 ,
wherein the objective lens satisfies a condition:
1.15 <f< 1.45 (2)
where f represents a focal length of the objective lens.
16. The optical system according to claim 13 ,
wherein the objective lens satisfies a condition:
1.20<( SAG 1)′/ n< 1.65 (3)
where (SAG1)′ represents a first derivative of the sag of the first surface at the inflection point and n represents a refractive index of the objective lens.
17. The optical system according to claim 13 ,
wherein the objective lens satisfies a condition:
−0.60<(( SAG 2)′/ n ) MIN ≦−0.10 (4)
where ((SAG2)′/n) MIN represents a minimum value of (SAG2)′/n, (SAG2)′ represents a first derivative of a sag of the second surface, and n represents a refractive index of the objective lens.
18. The optical system according to claim 13 ,
wherein the objective lens satisfies a condition:
0.77 <d /( f·n )<1.10 (5)
where f represents a focal length of the objective lens, d represents a lens thickness of the objective lens, and n represents a refractive index of the objective lens.
19. The optical system according to claim 13 ,
wherein the objective lens satisfies a condition:
−1.50 <r 1 /r 2<−0.60 (6)
where r1 represents a radius of curvature of the first surface, and r2 represents a radius of curvature of the second surface.
20. The optical system according to claim 13 , wherein the first surface is located on a light source side and the second surface is located on an optical disc side.
21. The optical system according to claim 13 ,
wherein the first surface has an anti-reflection coating formed thereon,
wherein the anti-reflection coating formed on the first surface satisfies a condition:
1.00 <Tp/Tc< 1.40 (7)
where Tc represents transmissivity of a circular central part defined within a range of 10 percent of the effective diameter with respect to a center of the first surface, and Tp represents transmissivity of a ring-shaped part defined outside of 90 percent of the effective diameter with respect to the center of the first surface,
wherein the objective lens satisfies a condition:
1.75<λ 0 /λ min <2.00 (8)
where λ 0 represents a wavelength at which reflectivity of the anti-reflection coating takes a minimum value for a beam incident perpendicularly on the anti-reflection coating, and λ min represents a minimum wavelength of wavelengths of laser beams to be used.
22. The optical system according to claim 13 , wherein the aspherical surface is expressed by the following equation:
X
(
h
)
=
Ch
2
1
+
1
-
(
1
+
κ
)
C
2
h
2
+
A
4
h
4
+
A
6
h
6
+
A
8
h
8
+
…
+
A
22
h
22
wherein X(h) represents the sag amount;
h is the height from the optical axis;
κ represents a conical coefficient;
A 4 , A 6 . . . A 22 are aspherical coefficients of 4 th , 6 th . . . 22 nd orders, respectively; and
C is curvature of the aspherical surface on the optical axis.